This invention relates to photoelectric sensing systems, as well as the photoelectric sensing units proper, and it relates more particularly to the provision of means for facilitating the adjustment of such systems or units. The invention provides for this purpose a contrast indicating arrangement and makes use of an instrument which gives a visual indication of the light level returned to the sensing unit as detected in the "light" state and the light level returned to the detector in the "dark" state. The difference between the maximum or "light" reading of the instrument and the minimum or "dark" reading of the instrument, thus is the contrast differential; by the same token the ratio between these two readings is the contrast ratio.
More specifically, the instrument proposed by the invention as the contrast indicator for use in photoelectric sensing systems is a bargraph instrument, that is an instrument of a type which in the last few years has made its appearance as a signal strength indicator or readout device in a number of applications such as radio receivers, tape recorders, automobile dashboard instruments, light meters in cameras and the like. The bargraph instrument consists essentially of a display arrangement in the form of an array of light emitting means such as LEDs (light emitting diodes) and an electronic control or driver circuit for this display. With the aid of this driver circuit the LEDs are incrementally illuminated--individually or cumulatively depending on the option chosen--in instantaneous response to variations, that is increases or decreases, in the input voltage to the instrument. Thus, if the LEDs are successively illuminated in increments of, say, one volt, the level of the input voltage in volts can be read off the instrument by simply observing which particular one in the array of LEDs or what total number of consecutive LEDs has been lit; and in the combination in which the bargraph instrument is used according to the invention, both a "light" reading and a "dark" reading can be obtained in this manner and thus the contrast between light and dark can be quantitatively determined at a glance during the operation of the system. The determination of the contrast is important in photoelectric sensing because the proper adjustment of the system is substantially facilitated by this determination.
An example is the case where a continuous flow of objects on a conveyor passing the photoelectric sensor is to be monitored, for instance for the purpose of counting the objects. In this application--which is only one of a large number of such instances--, as one after the other of the objects detected by the sensor breaks or makes the light beam, the reading of the LED display of the bargraph instrument will fluctuate between the "light" level and the "dark" level, and in this manner the contrast differential, that is the light range actually available in the detecting operation can be dynamically observed by direct readout, and the scanning or sensing system can be adjusted accordingly.
A precise adjustment of the photoelectric sensing system is of particular importance where the differential between "light" and "dark" is very small so that both the extent and the absolute level of the critically narrow range can be accurately determined. One such case for example is a system for use with an automatic envelope stuffing machine, which is called upon to photoelectrically detect whether, say, one or more papers of the same kind, such as bills, have been dropped in the same envelope by the machine.
Another such example is where it has to be photoelectrically determined whether a certain thin washer which may be only a few thousandths of an inch thick has, in fact, been placed on a potentiometer shaft in assembly--in spite of the aggravating circumstance that on the fully assembled shaft a white nylon area is present just below the washer. While the use of fiberoptics--in this example as well as in the immediately preceding one--is of some assistance in the solution of the problem, without resort to the invention it is extremely difficult to detect in this case the minute difference between the amount of light coming off the washer when it is indeed in place on the shaft, and the level of light which comes off the aforementioned white area when the washer itself is in fact not in place.
Another and most illustrative example for the great usefulness of the invention is the photoelectric detection of high motor speeds, usually for the purpose of directly controlling this speed. This photoelectric detection can be carried out by directing a beam of light at the end of the motor shaft having a flat area formed thereon, by means of which a pulley or the like can be mounted in the actual use of the motor. Since the light reflected by the flat area is different from the light reflected by the remainder of the periphery of the shaft end, the analog signal derived from the detected light signal of the photoelectric system will fluctuate at a very rapid rate, namely that corresponding to the rotational speed, say 500 revolutions per second, of the motor. The fluctuations are so fast that the human eye cannot follow them.
A conventional electromechanical volt meter, too, because of its inertia could not be used to give an indication of the difference in level of the returned light as between the two situations in question (cylindrical shaft surface on the one hand and the flat area on the other): The needle of a conventional volt meter would merely settle at a nominal but meaningless reading without permitting any conclusions with respect to the light range that can actually be utilized. A digital volt meter would not work either. In a conventional digital volt meter the voltage is sampled for a certain length of time and then displayed but the times involved are not long enough to allow the sampling and displaying to be done in anything like a 500ths of a second.
The bargraph type contrast indicator in the system according to the invention does make a reading of the two extreme conditions in question possible even though the shaft speed is of the high order of magnitude mentioned. What happens in this case is that because of the light fluctuations occurring in the range between the maximum light reading (say 8 volts) and the minimum light reading (say 4 volts) the LEDs in this 4-volt range therebetween appear to the eye to be dimly lit at a steady glow.
Therefore, notwithstanding the high rotational speed of the motor and thus the high rate of change in output of the sensor system between "light" and "dark", the contrast indicator according to the invention gives the user an indication of the width of the voltage range he has to work with (8-4=4 volts) which in this instance is enough to safely allow for drift, that is a change of parameters during operation; and also an indication at what level this range is located. In the present instance the midpoint (6 volts) of this range roughly coincides with the midpoint (5.5 volts) of the scale of the instrument, assuming an instrument having a 10-volt scale is used. If it does not, the controls of the DC amplifier of the sensor system, particularly its bias ("offset") control can be adjusted to relocate, that is to center, the level of the range correspondingly.
This is especially desirable if the sensor system is designed for digital operation, that is arranged to produce a switching signal when the analog voltage reaches a certain selected level. In this instance the switching point is preferably set to a level slightly above 5 volts--the midscale point of the instrument.
From the foregoing general description it will be clear that with the contrast indicator according to the invention the available "light/dark" contrast, that is the contrast differential, can be ascertained at a glance regardless of the rapidity with which the detected light changes between the two extremes, and that as a result the user is enabled to present the--preferably proportional--analog DC signal developed by the sensor system at the most desirable level, and he is also enabled to determine the feasibility of the sensing task at hand and thereby eliminate marginal performance.
In addition, the contrast indicator of the invention can also be applied by the user to physically, i.e., optically align the photoelectric sensing system with great sensitivity. In this connection it may be mentioned that alignment indicators for photoelectric sensors are known per se. U.S. Pat. No. 4,356,393 to Fayfield, for example, discloses such an alignment indicating apparatus in which a repetitive alignment indicating signal is produced whose frequency is a function of the intensity of the received light signal. The indicator itself is in the form of a light-emitting diode so that this LED flashes at a rate indicative of the degree of alignment of the sensing system. While rough alignment of the optical system is possible in this manner, the flashing-rate type of indication does not lend itself to the determination of the "light/dark" contrast in operation and, of course, is unusable for such a determination in situations where the changes between "light and dark" detection occur at a rate too high to be followed by the human eye.